Seismic Strengthening of RC Short Columns by EBR and NSM Methods Using CFRP Sheets and High Strength Steel

Existence of short columns in buildings and bridges is a serious challenge in earthquakes. This destructive phenomenon occurs due to the difference in length of the column at a certain level that is mainly because of architecture consideration, such as the placement of building on a slop or restriction of column with nonstructural walls and openings or difference in story level in structures because the existence of mezzanine floor.
Short columns have brittle shear failure in comparison with tall columns. This kind of failure causes a reduction in the energy dissipation capacity of the column. Shear failure is the most critical failure mode in RC short columns due to the none-observance of seismic details or sufficient transverse reinforcements against seismic loads. As concrete tensile stresses reach concrete tensile strength and the diagonal cracks appear, the concrete cover is detached and starts to shed. Then the failure and openings of transverse reinforcements and as a result the buckling longitudinal reinforcements occur. The above process leads to the disintegration of the core concrete and the sudden fracture and embrittlement of the column.
In externally bonded reinforcement by FRP composites, FRP materials are different from the materials of the RC (concrete and steel) parts. The use of FRP is limited to high temperatures and has a low resistance to fire. On the other hand, strengthening with FRP composite materials is economically expensive. Mostly, High Strength Steel (HSS) bars have been used in the design and construction of the RC structures and not in strengthening. Today, due to the growing population and increased demand for raw materials and energy, solutions have been taken to optimize standards and to save on consumables, production and cost reduction. Steel reinforcements are one of the most widely used building materials with a huge number of applications in a variety of structures. Due to the considerable cost of using steel in structures, the use of HSRs has been considered as one of the major options. The use of HSRs has economic justification because of reduced human resources, reduced consumption of materials, time and manufacturing efficiency, reduced environmental damage because of the optimal utilization of materials and reduced transportation costs. Because of the greater tensile strength of these bars than ordinary ones, it leads to a brittle failure in concrete prior to rebar flaking. It, therefore, limits their application in regions with high seismic hazard.
In this paper, with the modeling of nine RC short columns, without increasing the stiffness, their shear strength has increased with the help of composite and high strength steel. Two techniques were used to strengthen the diameter of the short columns against seismic loads. These techniques include EBR with FRP composite materials and NSM with HSS. The results show that in general, near surface mounted with high strength steel is more effective on increasing the dissipated energy and the ductility factor and externally bonded retrofitting is more effective on the increase of the load-displacement sub-curve and the peak load capacity.